1. Field of the Invention
The present invention relates to an automatic focusing device which performs an automatic focusing operation only when a main object is present within a range preset by a photographer.
2. Description of the Prior Art
An automatic focusing device usually focus an imaging lens based on focus information (image deviation, sharpness, etc.) on an object captured in a vicinity of an imaging field (hereinafter referred to as an object captured in a distance measurement view field). Accordingly if the object desired by the photographer (hereinafter referred to as a main object) is out of the center of the imaging field and cannot be captured in the distance measurement view field, the focusing operation is performed for an object which is different from the main object and captured in the distance measurement view field. In a single-lens reflex type camera which frequently uses a long focal length lens, the use of the automatic focusing device creates the following problems.
(1) Because of a high imaging magnification, it is difficult to continuously capture the main object in the distance measurement view field. Accordingly, the above defect remarkably appears.
(2) Because of a large amount of lens detection, the focusing time to the object captured in the distance measurement view field is long when the object which is significantly distant from the main object is captured in the distance measurement view field. Therefore, a long time is required to refocus the lens to the main object and a photographing chance may be lost.
In order to resolve the above problems, it has been proposed to classify the distance to the imaging object to a distant zone, an intermediate zone and a near zone, and the photographer designates one of those zones so that the focusing operation is performed within the designated zone.
For example, if the main object is located in the intermediate zone, the intermediate zone is previously designated and the focusing operation is performed while the main object is captured in the distance measurement view field. Even if the main object goes beyond the distance measurement view field, the focused distance is limited within the zone so that the focusing operation for the object which is significantly distant from the main object is prevented, to assure an acceptable focus condition to a photograph of the main object.
However, in this method, it is necessary to predict the zone of the main object. This requires a decision (distance prediction) which is contrary to the purpose of the automatic focusing device. Further, if the main object is located at a boundary of the zones and the main object moves even slightly when the main object has been captured in the distance measurement view field, the main object cannot be exactly focused.
For example, if the main object is located at the boundary of the distant zone and the intermediate zone and the distant zone is designated, and if the main object goes beyond the distance measurement view field, the significant defocused state for the main object is prevented because the focusing operation is permitted within the distant zone, but if the main subject moves into the intermediate zone while the main object is captured in the distance measurement view field, the exact focusing for the main object can no longer be performed.
As an approach to resolve the above problem, the inventor of the present invention has proposed to provide a control circuit which permits the movement of the lens within an allowable range for a preset distance so that the exact focusing operation is performed for the movement of the main object within the allowable range for the preset distance when the object is captured in the distance measurement view field, and if the main object goes beyond the distance measurement view field, the movement of the lens is inhibited at the limit position of the allowable range to prevent a significantly defocused state relative to the main object which is within the allowable range.
However, although the allowable range can be preset, it is difficult to determine the allowable range when the range of the object movement distance is very much larger. If the allowable range is set too narrowly, the imaging lens is stopped at the limit position of the allowable range if the object moves beyond the allowable range and the exact focusing operation is not attained in spite of the fact that the automatic focusing apparatus produces an exact focusing signal.
Since the depth of focus of the imaging lens is determined by a focal distance and a stop value, a high degree of consideration and experience are required to set the distance while taking the allowable range into consideration.